The present invention relates in general to electrical motors and in particular to motors in which windings are excited with time spaced energizing pulses. More particularly, the invention has to do with accommodation of inductive flyback energy when motor windings excited by current flow from a dc. voltage source are turned off by a series switching device.
Throughout this specification reference will be made to brushless dc. motors in which the magnetic field is created by permanent magnets on the rotor. The basic construction and operation of such motors is well known to those skilled in the art. It should be understood, however, that a straightforward equivalent of a permanent magnet rotor may be created by field windings carried on a permeable rotor and appropriately excited with current via slip rings to create the necessary magnetic field across the rotor-stator gap.
In electrical motors of the type wherein stator windings are pulsed by periodic closing and opening of switch means to draw current from a dc. voltage source, inductive "kick" or flyback action creates a reversed polarity voltage across a winding when the switch means opens and exciting current is interrupted. Unless appropriate accommodations are made, the flyback voltage may create breakdown damage to the switch means which are usually constituted by on-off controlled transistors. Moreover, unless flyback energy is dissipated or utilized within critical time spans, it may detract from useful output torque from the motor. Many flyback circuits heretofore employed in the prior art have added to the expense of a complete motor and its associated control apparatus and/or reduced the efficiency with which electrical energy from a power source is converted into mechanical energy.